The present invention relates to a down hole hammer.
Down hole hammers usually utilise compressed air to effect reciprocation of the hammer in order to effect their hammering action. In order to improve the performance of hammers it has been a characteristic of such hammers that the air which has been delivered to the hammers has increased in pressure significantly over the years. However, the hammers according to previous designs have not significantly changed in design to accommodate for the increased pressure and as a result the hammers have failed with this increased pressure due to the resultant increased stresses that are created within the hammer. In addition previous hammers have not fully utilised the energy contained in the pressurised fluid delivered to them.
The prior art to the invention is exemplified by U.S. Pat. Nos. 4,084,646, 4,790,390, 4,819,746 and AU-B-40793/96. The difficulty with the high fluid pressures which are currently being applied to hammers relates to the manner in which the pressurised fluid is to be introduced into the hammer to achieve the greatest effect but with the minimum of fluid. It is conventional practice as disclosed in each of the above specifications to deliver fluid into the hammer to effect movement of the piston to the impact position when the piston is at its fully raised position. As a result the fresh injection of pressurised fluid id effected when the piston is decelerating, changing direction and accelerating. This does not comprise an efficient use of the energy available from the pressurised fluid and results in the relevant ports being open for a greater period of time than is necessary. In addition it is important that the lubricant which is entrained in the pressurised fluid is delivered directly to the surfaces of the hammer which are in sliding engagement and this has not been found to be the case in at least some of the prior art. In addition many of the prior art hammers have provided the valving required for operation of the hammer by providing ports through the body of the hammer and/or providing counterbores in the ends of the central bore of the piston. It has been found that this feature can result in structural weaknesses in the piston and reduces the mass of the piston which reduces the drilling capacity of the hammer.
It is an object of this invention to provide a down hole hammer which at least partially overcomes some of the difficulties of the previous hammers.
Accordingly the invention resides in a down hole hammer comprising a cylindrical casing having a top sub at one end which is adapted to be mounted to a drill string and a drill bit support at the other end to define a piston chamber, said drill bit support being adapted to receive a drill bit, a piston received within the piston chamber for reciprocation between the one end and the other end, said piston being formed with a central portion which is slidingly and sealingly engaged with the internal wall of the casing, said central portion providing a first shoulder at the end proximate the top sub, said piston having one end portion which is intended to impact on the drill bit on movement of the piston to the impact position and the one end portion being configured to be sealingly engaged with the drill bit support and/or the internal face of the piston chamber when the piston is in the impact position to define a first space between the internal wall of the piston chamber, the one end portion and the drill bit support, a portion of the length of the internal wall of the casing adjacent the top sub being provided with two sets of fluid delivery ports at locations spaced axially along the portion of the length of the casing, the external radial face of the piston and or the radial face of the piston chamber being formed to enable the piston to cooperate with the internal wall of the casing and with a first set of said fluid ports to admit fluid into the first space when the piston is in the impact position to cause movement of the piston to a raised position at which position the piston is in spaced relation from the drill bit support, the external radial face of the piston and or the radial face of the piston chamber being formed to enable the piston to cooperate with the internal wall of the casing and with the second set of said set of ports to admit fluid into a second space between the piston and the top sub when the piston is adjacent the top sub to cause movement of the piston to the impact position, wherein the admission of fluid to the second space is at least reduced as the piston moves to the fully raised position.
According to a preferred feature of the invention the drill bit support further comprises, in part, an annular bush supported in the casing to extend into the piston chamber from the drill bit support, the one end portion of said piston having a reduced diameter to provide a second shoulder at the proximate end of the central portion, the area of the second shoulder being greater than the area of the first shoulder, the one end portion being configured to be sealingly engaged with the annular bush when the piston is in the impact position to define the first space.
According to a preferred feature of the invention the portion of the length of the internal wall of the casing is defined by an inner sleeve extending from the top sub in the direction of the drill bit support and providing a plenum space between the sleeve and the casing which is in communication with fluid being delivered from the top sub, said sets of fluid ports being provided in the sleeve. According to a preferred feature of the previous feature the inner sleeve provides a portion of the internal face of the piston chamber which is of reduced diameter.
According to a preferred feature of the invention the other end portion of the piston is of a reduced diameter and is formed with at least two annular ribs at axially spaced locations along the other end portion which are slidably and sealingly engaged with the inner sleeve wherein a third space is defined between the radial face of the piston chamber, the radial face of the piston, the innermost rib and the first shoulder. According to a preferred feature of the invention the third space is able to open into the first space when the piston is at or in the vicinity of the impact position. According to a preferred form of this feature of the invention the external radial face of the piston and the internal radial face of the piston chamber are formed to provide cooperating formations which control the flow of fluid between the third and first space with relative movement between the piston and the piston chamber. According to one embodiment the innermost rib is located to cooperate with the first set of ports to control the delivery of fluid to the third space wherein fluid flow to the third space is closed as the piston approaches the raised position. According to an alternative embodiment the first set of ports are in constant communication with the third space. According to a preferred feature of the previous preferred form the cooperating formations in the surface of the piston and the piston chamber comprise a plurality of flute-like recesses formed on the surface of the central portion which extend from the first shoulder to an intermediate location on the central portion and one or more recesses formed in the internal face of the casing. According to an embodiment of the previous preferred feature the axes of the flute-like recesses are angularly offset from the longitudinal axis of the piston to substantially the same extent and in substantially the same direction.
According to a preferred feature of the invention the outermost rib cooperates with the second set of ports to control the delivery of fluid to the second space wherein said outermost rib is located to cooperate with formations in the inner face of the piston chamber to admit fluid from the second set of ports to the second space during the movement of the piston from its impact position to its raised position and one of said annular ribs engage with the second set of ports to at least restrict the flow of fluid to the second space as the piston approaches its fully raised position. According to one preferred embodiment two annular ribs are provided on the other end portion and the one of said annular ribs comprises the innermost set of ribs. According to another embodiment three annular ribs are provided on the other end portion and the intermediate rib comprises the one of said annular ribs. According to a preferred feature of the previous embodiment the one of said annular ribs closes the first set of ports from communication with the third space on the drill bit being axially displaced outwardly from the impact position.
According to a further preferred feature the piston is formed with a central bore which cooperates with a central tubular element extending into the piston chamber from the top sub to be slidably and sealingly engaged therewith during the movement of the piston from its impact position to its raised position wherein the central bore enables fluid to be exhausted from the second space when the central bore is out of engagement with the central tubular element. According to one embodiment the hammer is a reverse circulation hammer and the central tubular element is provided with an axial extension extending into the drill bit, said axial extension not being in sealing engagement with the central bore. According to an alternative embodiment the central tubular element is provided with an outlet at its outer end to enable a restricted flow of fluid into the bore of the piston to be exhausted from the drill bit to be supported by the drill bit support and wherein the drill bit supports a foot valve which is sealingly and slidably engaged by the bore when the piston is at its impact position and for a portion of the movement of the piston from the impact position to the raised position.
The invention will be more fully understood in the light of the following description of three specific embodiments.